(1) Field of the Invention
The present invention relates to a line circuit for driving a ringer and a service indicating lamp in a telephone set. More particularly, it relates to a line circuit of a compact design employing a high-frequency transformer for supplying both a ringing signal and a lamp driving signal.
(2) Description of the Related Art
As an example of a service indicating function, the message waiting function is explained. By using a telephone set having the message function, when a called party is absent, a message lamp for message waiting function provided on the telephone set is lit to inform the later arriving party that someone has called him. For example, in a hotel, when a message is left at the front desk during the absence of a guest and this message should be conveyed to the guest when he returns to his room, a switch provided on the front desk is operated so that the message lamp of the telephone set in the guest room is lit, informing him that a message has been left at the front desk. By this, the guest can know that there is a message for him. The lamp can be lit not only by the switching operation at the front desk but also by dialing a special number from another telephone set, an exchange in the hotel decoding the special number so as to light the message lamp.
To light the message lamp, a direct-current (D.C.) voltage of, for example, 100 V must be applied through a subscriber line to the message lamp.
Apart from the message lamp, in a two-wire subscriber line, to ring the ringer in the telephone set when a call is received, a low-frequency ringing signal of, for example, 16 Hz is sent from a line circuit to the subscriber line. The root mean square value of the ringing signal is, for example, 75 V.
Conventionally, there are three power supplies, i.e., a first power supply for supplying a voltage of, for example, -48 V, for communication to the subscriber line, a second power supply for supplying a ringing signal to the ringer, and a third power supply for supplying a message signal to the message lamp.
Further, to generate the low-frequency ringing signal having a root mean square value of 75 V as an example, a low-frequency transformer, which is large in size and heavy due to the low frequency, is conventionally employed.
To generate the message signal of D.C. voltage, a separate rectifier circuit is conventionally employed by which a commercial alternating current is rectified to obtain a D.C. voltage of, for example, 100 V. This increases the number of parts.
Therefore, the conventional line circuit, if it includes a ringer driving circuit and a lamp driving circuit, has disadvantages of large size and heavy weight, causing problems in installation.